1. Field of the Invention
The present invention relates to an apparatus for detecting the rotational position (crank angle) of the crankshaft of an internal combustion engine. More particularly, this invention relates to a crank angle detecting apparatus suitable for detecting the crank angle of an internal combustion engine having a plurality of cylinders.
2. Description of the Related Art
A reciprocating engine has a plurality of pistons retained in respective cylinders in a reciprocatable manner, and a crankshaft. The crankshaft has a plurality of cranks associated with the respective pistons. Each piston is coupled to the associated crank by a connecting rod. The reciprocation of each piston is converted to the rotary motion of the crankshaft via the connecting rod. The position of the piston in each cylinder correlates with the rotational position of the crankshaft. Therefore, the position of the piston in each cylinder of the engine can be discriminated by detecting the rotational position (crank angle) of the crankshaft.
Various timing controls including the ignition timing control and fuel injection timing control for an engine are executed based on the crank angle (denoted by ".degree. CA") that is detected by a crank angle detecting apparatus.
Japanese Unexamined Patent Publication No. 5-288112 discloses this type of crank angle detecting apparatus. This detecting apparatus comprises an engine speed sensor disposed near a crankshaft and a timing sensor disposed near a cam shaft. The engine speed sensor has a crank rotor, which rotates together with the crankshaft, and an electromagnetic pickup provided to face the outer surface of the rotor. The crank rotor has a plurality of projections provided on its outer surface at intervals of 30.degree. CA. The crank rotor further has a gap portion formed on its outer surface by removing one projection. The timing sensor has a cam rotor, which rotates together with the cam shaft, and an electromagnetic pickup provided to face the outer surface of this rotor. The cam rotor has a single projection on its outer surface. Every time this projection passes the pickup of the timing sensor, or every time the cam rotor rotates 360.degree., the pickup sends a timing signal to a controller. As the cam rotor rotates 360.degree., the crankshaft rotates 720.degree. CA.
When the aforementioned gap portion passes in front of the pickup of the engine speed sensor, the pickup sends a reference position signal to the controller. When this reference position signal is output, the crankshaft is positioned at a specific reference position. Thereafter, every time a projection of the crank rotor passes the pickup of the engine speed sensor, the pickup sends a pulse signal to the controller. The controller counts the number of the pulse signals and determines if the timing signal from the timing sensor has been input when the count value reaches a predetermined value. Then, the controller acquires a crank angle corresponding to a specific cylinder based on the result of the determination. The time at which the count value reaches the predetermined value follows soon after the generation of the reference position signal. That is, the time of the generation of the timing signal comes soon after the generation of the reference position signal.
As apparent from the above, the provision of the two sensors, the engine speed sensor and the timing sensor, allows the crank angle corresponding to a specific cylinder of a multi-cylinder engine to be detected. Based on the detected crank angle, the ignition timing control, fuel injection timing control and the like associated with each cylinder are properly carried out. The timing signal is generated very soon after the generation of the reference position signal. When cranking causes the crankshaft to rotate to a position slightly over the reference position corresponding to the generation of the reference position signal, a crank angle corresponding to a specific cylinder can be certainly detected.
The reference position signal is output from the engine speed sensor every time the crankshaft makes a rotation of 360.degree. CA. When the engine stops with the gap portion of the crank rotor beyond the associated pickup, the first reference position signal is sent out from the engine speed sensor after cranking causes the crankshaft to turn approximately 360.degree. CA. In other words, until cranking causes the crankshaft to turn approximately 360.degree. CA, a crank angle corresponding to a specific cylinder cannot be detected. In an engine that employs a distributor-less ignition system, unless a crank angle corresponding to a specific cylinder can be detected, a cylinder to be ignited and the timing for igniting that cylinder cannot be determined and ignition cannot be initiated. This delays the start of the engine.
As a solution to the above problem, two gap portions may be provided on the outer surface of the crank rotor at an interval of 180.degree. CA and two projections may be provided on the outer surface of the cam rotor at an interval of 180.degree. CA. This structure however disables the controller's ability to determine with which one of the two gap portions the reference position signal received from the engine speed sensor is associated. Further, when having received the timing signal from the timing sensor, the controller cannot determine with which one of the two projections the signal is associated. It is therefore not possible to detect a crank angle corresponding to a specific cylinder.